Known gas turbines include a compressor section, a combustion section and a turbine section. For example, Prior Art FIG. 1 depicts a typical industrial gas turbine engine 10 comprising in axial flow series: an inlet 12, a compressor section 14, a combustion section 16, a turbine section 18, a power turbine section 20 and an exhaust 22. The compressor section 14 is driven by the turbine section 18 through a common shaft connection. The combustion section 16 typically includes a circular array of a plurality of circumferentially spaced combustors. A fuel or fuel mixture is burned in each combustor to produce a hot energetic flow of gas, which flows through a transition piece for flowing the gas to the turbine blades of the turbine section.
The primary air pollutants produced by gas turbines are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. For many years now, the typical combustor has included a primary injection system at a front end thereof to introduce fuel into the combustion chamber along with compressed air from compressor section 14. Typically, the fuel and air are premixed and then introduced into an igniter to produce a flowing combustion stream that travels along a length of the combustion chamber and through the transition piece to the first row of turbine blades. One challenge in such single site injection systems is there is always a balance to be obtained between the combustion temperature and the efficiency of the combustor. While high temperatures generally provide greater combustion efficiency, the high temperatures also produce higher levels of NOx. Moreover, the combustion of the primary fuel typically forms a flame having a temperature profile in the combustion chamber and transition piece that has a relatively hot core temperature and cooler peripheral zones. In these cooler peripheral zones, efficiency of combustion is typically less than that of a hotter central zone. The hot core temperature typically has increased levels of NOx due to the high temperatures therein. Further, within the cooler peripheral areas, there may be found increased levels of carbon monoxide and unburned hydrocarbons due to the sub-optimal combustion temperature.
More recently, combustors have been developed that also introduce a secondary fuel into the combustor. For example, U.S. Pat. Nos. 6,047,550, 6,192,688, 6,418,725, and 6,868,676, all disclose secondary fuel injection systems for introducing a secondary air/fuel mixture downstream from a primary injection source into the compressed air stream traveling down a length of the combustor. While the introduction of the fuel at a later point in the combustion process appears to be able to reduce at least some NOx levels due to the short residence time of the added fuel in the transition piece and by maintaining a lower combustion temperature by adding less fuel at the head end, there still remains a hotter central zone and cooler peripheral zones in the combustion chamber and transition piece. The cooler peripheral areas have decreased combustion efficiency and increased levels of carbon monoxide and unburned hydrocarbons. Further, raising the temperature of the cooler peripheral regions to an optimal temperature for combustion necessarily requires increasing the temperature of the hotter central region to a temperature that likely produces higher NOx levels.